Dinosaur Footprint Asemblage from the Lower Cretaceous Khok Kruat Formation, Khorat Group, Northeastern Thailand

Accepted Manuscript

Dinosaur footprint asemblage from the lower Cretaceous Khok Kruat Formation, Khorat Group, northeastern Thailand

Shohei Kozu, Apsorn Sardsud, Doungrutai Saesaengseerung, Cherdchan Pothichaiya, Sachiko Agematsu, Katsuo Sashida

PII: S1674-9871(17)30032-4 DOI: 10.1016/j.gsf.2017.02.003 Reference: GSF 539

To appear in: Geoscience Frontiers

Received Date: 30 November 2016 Revised Date: 19 January 2017 Accepted Date: 3 February 2017

Please cite this article as: Kozu, S., Sardsud, A., Saesaengseerung, D., Pothichaiya, C., Agematsu, S., Sashida, K., Dinosaur footprint asemblage from the lower Cretaceous Khok Kruat Formation, Khorat Group, northeastern Thailand, Geoscience Frontiers (2017), doi: 10.1016/j.gsf.2017.02.003.

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Dinosaur footprint asemblage from the lower Cretaceous Khok Kruat Formation, Khorat

Group, northeastern Thailand

Shohei Kozu a, *, Apsorn Sardsud b, Doungrutai Saesaengseerung b, Cherdchan Pothichaiya b,

Sachiko Agematsu a, Katsuo Sashida a

a Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki

305-8572, Japan b Department of Mineral Resources, Ministry of Natural Resources and Environment,

Thailand MANUSCRIPT Corresponding author. E-mail address: [email protected], Tel:

+81-29-853-6138

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Abstract

The Khok Kruat Formation is the upper part of the Khorat Group, which consists of upper Lower Cretaceous non-marine sedimentary rocks in northeastern Thailand. Many dinosaur footprints have been known from the upper Lower Cretaceous (Aptian–Albian)

Khok Kruat Formation at the Huai Dam Chum (Tha Uthen) site, northeastern Thailand.

Approximately 600 tracks occur in thin mudstone layer of the northern part of the outcrop at the Huai Dam Chum track site. Two types of footprints, small-sized theropod and crocodylomorph are imprinted with mud cracks and ripple marks on the thin mud layer.

Most of footprints are referred to cf. Asianopodus , and are imprinted by small-sized MANUSCRIPT theropoda, probably ornithomimosauria. Theropod tracks are mainly separated into two groups, Group A and Group B. From ichnological viewpoints, the small-sized theropod track assemblage indicates the herd behaviour and its idiosyncratic group composition. In particular, the histogram of size-frequency measurements of Group A shows the anomalous bimodal distribution. We consider that there are two hypotheses; the first one is due to the male-female difference,ACCEPTED and the second is a result of the different growing stage.

Keywords: Dinosaur footprint assemblage; Cretaceous; Khorat Group; Khok Kruat

Formation; Thailand

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1. Introduction

Gregarious behaviour has been suggested for a number of dinosaur taxa, including ceratopsids, ornithopods, theropods, and sauropods (Myers and Fiorillo, 2009). Such behaviour is known from multiple examples of skeletal evidence and from abundant footprint evidence (Gillette and Lockley, 1989; Lockley, 1991). However, at present, most of the firm evidence of gregarious behaviour is provided by the ichnological record, with many tracksites exhibiting signs of group behaviour (table 1 in García-Ortiz and

Pérez-Lorente, 2014). The footprint record provides a great deal of information about the MANUSCRIPT herds of imprint producers that is not available in the bone fossil record, including movement speed, style of gregarious behaviour, herd structure, and the organigram within a group (e.g. Gillette and Lockley, 1989; Lockley, 1991). Gregarious behaviour has been confirmed in sauropod track assemblages (e.g. Lockley et al., 1994, 2012) and ornithopod track assemblages (e.g. Ostrom, 1972; Currie, 1983; Fiorillo et al., 2014). Similarly, gregarious behaviourACCEPTED was common in small bipedal dinosaurs (Lockley and Matsukawa,

1999).

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The Upper Triassic to Lower Cretaceous Khorat Group, which consists of non-marine sedimentary rocks, crops out widely in northeastern Thailand (Fig. 1). Many fossils such as dinosaur bones, dinosaur footprints, fish, crocodilians, turtles, bivalves, and palynomorphs have been recovered from the Khorat Group (Fig. 2) (Meesook, 2011; Meesook and

Saengsrichan, 2011). At the Huai Dam Chum dinosaur tracksite, which is located on the eastern edge of northeastern Thailand, a number of dinosaur tracks formed by small-sized bipedal dinosaurs are present in outcrop. Dinosaur footprints have previously been reported from this area (Buffetaut et al., 2005; Le Loeuff et al., 2005, 2009; Sato and Tumpeesuwan,

2005; Matsukawa et al., 2006); however, there has been no previous study of the MANUSCRIPT ichnotaxonomic classification or quantitative commu nity analysis of the dinosaur footprint assemblage. In this study, we systematically describe tracks and conduct quantitative analysis of theropod tracks from the upper Lower Cretaceous Khok Kruat Formation of northeastern Thailand.

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2. Geological setting

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The Upper Triassic to Lower Cretaceous non-marine sedimentary rocks exposed in northeastern Thailand are referred to as the Khorat Group (Ward and Bunnag, 1964).

Buffetaut et al. (1993) subdivided this group into eight formations, which are (from oldest to youngest) the Huai Hin Lat, Nam Phong, Phu Kradung, Phra Wihan, Sao Khua, Phu Phan,

Khok Kruat, and Maha Sarakham formations (Fig. 2). However, there is still considerable debate about the age, internal stratigraphic relationships, and depositional environment of the group (e.g. Racey et al., 1994).

The Khorat Group has yielded many dinosaur bone fossils; footprints have also been reported from several areas such as Phu Faek and Tha Uthen on the Khorat Plateau (e.g. Le MANUSCRIPT Loeuff et al., 2005, 2009). Knowledge of the East Asian dinosaur fauna has been improved by these studies. Dinosaur footprints at the Phu Luang site were first reported by Buffetaut et al. (1985a); this pioneering work was followed by many studies of dinosaur ichnology in northeastern Thailand (e.g. Buffetaut et al., 1985b, 1997, 2005; Buffetaut and Suteethorn,

1993; Polahan and Daoperk, 1993; Le Loeuff et al., 2002, 2003, 2005, 2007, 2008, 2009;

Lockley et al., 2002,ACCEPTED 2006b, 2009).

The Khok Kruat Formation, which is one of the upper units of the Khorat Group, is widely distributed in the foothills of the Phu Phan Range. In general, the formation consists

ACCEPTED MANUSCRIPT of fluvial deposits of mainly reddish-brown fine- to medium-grained sandstone, conglomerate, siltstone, and mudstone (Meesook, 2011). The Khok Kruat Formation is conformably underlain by the Phu Phan Formation and is unconformably overlain by the

Maha Sarakham Formation. The fresh-water shark Thaiodus ruchae has been recovered from the Khok Kruat Formation and also from the Takena Formation of the Lhasa block of

Tibet, the latter having been dated as Aptian–Albian on the basis of foraminiferal fossils

(Cappetta et al., 1990). From borehole samples, Sattayarak et al. (1991) suggested an Aptian age for the upper part of the Khok Kruat Formation (Racey and Goodall, 2009).

Meesook (2011) indicated that the depositional environment of these rocks was MANUSCRIPT meandering rivers, although less mature than the rivers that deposited the Sao Khua

Formation.

3. Occurrence of tracks

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The tracks described herein are preserved at the Huai Dam Chum site (N17°71'30.01",

E104°38'15.76"), Ban Lao Nat, Tha Uthen District, Nakhon Phanom Province, northeastern

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Thailand (Fig. 1). The Khok Kruat Formation crops out at the Huai Dam Chum site (e.g. Le

Loeuff et al., 2003). The outcrop, which consists mainly of very-fine-grained sandstone, is exposed along route 212 (Fig. 3A). Footprints are imprinted on the upper surface of a thin mudstone layer. The succession at this site (Fig. 4) consists of pinkish-brown sandstone with parallel laminations (~25 cm thick) overlying the footprint-bearing thin mudstone layer, which contains mud cracks and ripple marks. Pinkish-brown fine-grained sandstones with cross-laminations (5–15 cm thick) and pinkish-brown fine-grained sandstones with wavy-parallel laminations (15–25 cm thick) are observed in the lower part of the section.

Reddish-brown fine-grained sandstone, with parallel laminations in its lower part and MANUSCRIPT wavy-parallel laminations in its upper part (~65 cm thick in total), is underlain by pinkish-brown fine-grained sandstone (~30 cm thick) in the middle part of the section. In the upper part of the section, pinkish-brown fine-grained sandstone with wavy-parallel laminations, often discontinuous, is intercalated with white medium-grained sandstone with wavy-parallel laminations (10–15 cm thick). The stratigraphically uppermost part of the section consistsACCEPTED of pinkish-brown fine-grained sandstone (~60 cm thick). On the footprint-bearing thin mudstone layer, current-ripple marks that show a NW flow direction and mud cracks are overprinted by footprints (Figs. 5 and 6).

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The footprint-bearing outcrops are covered by an artificial roof for their protection and are easily accessed to observe footprints. The outcrop can be roughly separated into southern, middle, and northern parts (Fig. 3B). The area in which abundant footprints were found was originally a quarry; footprints were first reported by Le Loeuff et al. (2003).

Subsequently, there have been other reports of footprint in this area (Buffetaut et al., 2005;

Le Loeuff et al., 2005, 2009; Sato and Tumpeesuwan, 2005; Matsukawa et al., 2006). Le

Loeuff et al. (2003) reported more than 40 small-sized footprints (80 to 135 mm in length) on two large slabs at the Tha Uthen site. They estimated the trackmakers to have been small-sized theropods, and indicated the presence of deinonychosaurs. In 2005, they MANUSCRIPT described a large assemblage of small theropod tracks (Le Loeuff et al., 2005). Sato and

Tumpeesuwan (2005) also reported more than 100 footprints of small-sized theropods from the quarry in the same area at Tha Uthen. Those theropod tracks are generally of the same size and morphology as those at the Huai Dam Chum track site. In the northern outcrop,

~600 dinosaur footprints are imprinted in the thin mudstone layer. In this study, we measured dinosaurACCEPTED footprints in the northern part of the outcrop at the Huai Dam Chum track site.

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The total area of the outcrop of the track-bearing bedding surface is 72 m 2 in the north part of the Huai Dam Chum site (Fig. 5). Mainly, the tracks of small-sized theropod were recognized on the same bedding plane. On the basis of the track directions, the theropod tracks can be separated into two groups, Group A (tracks indicating a NW movement direction) and Group B (NE movement direction). As shown in Figs. 5 and 6, a total of 584 discrete theropod tracks were mapped, and 79 well-defined trackways (Group A: 66, Group

B: 13) consisting of 341 tracks (Group A: 300, Group B: 41) were clearly recognized. The remaining 243 theropod tracks were either isolated or too closely clustered to recognize individual trackways. The ichnological measurement data of the theropod tracks of Groups MANUSCRIPT A and B are similar (Table 1, 2). Thus, we consider that the theropod tracks of both groups were formed by the same type of small-sized theropod. On the other hand, two well-defined probably theropod trackways consisting of 8 flattened tracks were also recognized on the same bedding plane, and they shows S to SE movement directions. Many indistinct small-sized tetrapod tracks (length ~4.5 cm) occur on bedding surfaces in the formation, running across orACCEPTED parallel to the theropod trackways at the northern part of the outcrop (Figs.

5 and 6). Le Loeuff et al. (2005, 2009) regarded them as crocodile tracks. In our investigation, we could not capture the ichnological characteristics of the tracks precisely,

ACCEPTED MANUSCRIPT because of their poor preservation. The manus tracks are smaller than pes tracks. Pes tracks are elongate with indistinct elongated digit impressions directed anteriorly. The length of outer digit impressions is long comparatively. Thus, the tracks are neither lizard-like footprint nor dinosaur footprint. We tentatively refer those as crocodylomorph tracks, although the taxonomic designation of the tracks will be restudied eventually. These observations may suggest that gregarious theropods, a few solitary theropods, and crocodylomorphs travelled along the side of a river, which was probably meandering (Fig.

6).

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4. Systematic ichnology

4.1. Theropod tracks

Theropod Marsh, 1881

Asianopodus Matsukawa, Shibata, Koarai and Lockley, 2005

ACCEPTED cf. Asianopodus isp. (Fig. 7)

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Material: At least 79 trackways composed of 341 consecutive tracks in total, as the remaining 243 theropod tracks were isolated. The original tracks and trackway remain in the field (Figs. 5 and 6).

Locality and horizon: Khok Kruat Formation, Lower Cretaceous. Huai Dam Chum site,

Ban Lao Nat, Tha Uthen District, Nakhon Phanom Province, Thailand (N17°71 ′30.01 ″,

E104°38 ′15.76 ″).

Description: In the northern part of the outcrop, specimens T8n4, T8n5, T11n3, T14n2,

T14n4, T32n5, and T84n3 are well-preserved pes impressions that are sub-symmetrical, tridactyl small-sized tracks with slender digit impressions (Fig. 7; Table 1). In general, the MANUSCRIPT digit III impression is directed anteriorly and is longest, whereas that of digit II is shorter than that of digit IV. Digit II has two phalangeal pad impressions; digits III and IV have three phalangeal pad impressions. There is a distinct claw mark at the tip of each digit. The region and outline of the metatarsophalangeal pad impression are indistinct. The region also lies nearly in line with the axis of digit III. In well-preserved tracks T1–11, T14, T32 and

T84, the interdigitalACCEPTED angles between digits II and III are almost equal to those of digits III and IV. The interdigital angle between digits II and IV is 35°–63° (mean 50.7°).

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A total of 79 well-defined theropod trackways (T1–18, 20–22, 24, 27–34, and 38–84) were recognized on the northern outcrop (Table 2). Essentially, a “trackway” is composed of more than three consecutive tracks (Thulborn, 1990); in this study, we use “trackway” to mean more than two consecutive tracks for descriptive purposes. The mean footprint length and width are 13.6 cm and 10.1 cm, respectively, and the mean length/width ratio (L/W) is

1.35, indicating moderate mesaxony. The mean step, stride, and pace angulation are 65.6 cm, 131.9 cm, and 172.6°, respectively.

Comparison and discussion: Tridactyl tracks of the type that occur at the Tha Uthen site were typically made by bipedal theropods. Many theropod tracks have been described from MANUSCRIPT the Khorat Group (e.g. Buffetaut et al., 1985a, b, 1997; Le Loeuff et al., 2007, 2008).

Lockley et al. (2002, 2006b) described Siamopodus khaoyaiensis , which represents small- to medium-sized gracile theropods from the Khao Yai site, from the Lower Cretaceous strata of the Khorat Group. This ichnospecies has a length range of 14–30 cm and a width range of 11–25 cm; in addition, S. khaoyaiensis has a sub-symmetric bilobed heel. From an ichnological viewpoint,ACCEPTED the Tha Uthen specimens are different from S. khaoyaiensis .

In size, the Tha Uthen theropod tracks (mean footprint length 13.6 cm) are similar to

Grallator , which is a “brontozoid ichnite”. However, according to Hitchcock (1858),

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Grallator is characterized as a small (<15 cm) bipedal, functionally tridactyl ichnite, and is also more narrow (length/width ratio near or greater than 2). This difference means that the

Tha Uthen specimens are not referred to brontozoid ichnites such as Grallator (Fig. 7).

Xing et al . (2011) reported dinosaur footprint assemblage from the Upper Jurassic–Lower

Cretaceous Tuchengzi Formation, Hebei Province, China. In the lower part of the track site, small-sized tracks are referred to Therangospodus isp. (Fig. 7). The specimens are tridactyl theropod tracks with distinct claw marks, and reveal footprint length/width ration 1.3. In the specimens, discrete borders separate the metatarsophalangeal pad from digit traces II and III, but not digit IV. On the other hand, each proximal end of digit traces II, III and IV is MANUSCRIPT separated from the metatarsophalangeal pad impressi on in the Tha Uthen specimens. The morphology of the “heel” (metatarsophalangeal pad of digit IV) is an important characteristic in theropod tracks (e.g. Xing et al., 2014b). Thus, the Tha Uthen specimens are not referred to Therangospodus .

Azuma et al. (2006) made a report of more than a thousand dinosaur footprints in the

Lower CretaceousACCEPTED of the Ordos Plateau, Inner Mongolia, China. In total six different types of footprints (Footprint Type 1 to 6) are represented in Site I, II, and III. From the Site II, many small-sized tracks labelled as Footprint Type 6 are found. They do not have distinct

ACCEPTED MANUSCRIPT toe impressions and are teardrop-shaped, however, the trackmaker is estimated as theropod because of the typical gaits such as narrow trackways. The type 6 footprints are similar to the Tha Uthen specimens in size, however it is difficult to identify the internal structure because of its poor preservation of the type 6 footprints.

Zhang et al. (2006) reported the track assemblages from the Lower Cretaceous of Gansu

Province, China. In the main site, the unnamed small-sized tracks, Morphotype 2 (footprint length from 15 to 20 cm, narrow digit divarication) are briefly presented (Fig. 9; Zhang et al., 2006). The ichnological characteristics of Morphotype 2 tracks are not in agreement with Tha Uthen specimens. Li et al. (2006) also reported vertebrate track sites, from the MANUSCRIPT Lower Cretaceous, Gansu Province. Three different types of theropod tracks are represented from the site 1 and 2. In particular, Type 2 and 3 tracks are basically tridactyl with digit impressions II to IV. However, Type 2 and 3 tracks are tentatively referred as ichnogenus

Changpeipus and Grallator , respectively. Some of Type 2 tracks Changpeipus show traces of digit I behind the digit II impression. Type 3 tracks Grallator show the footprint length/width ratioACCEPTED 0.6 which is lower than that of Tha Uthen specimens. Thus, these Gansu tracks are different from the Tha Uthen specimens in morphology.

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Matsukawa et al. (2005, 2006) illustrated track-bearing slabs at a locality near Lao Nat (=

Huai Dam Chum site), and mentioned that those specimens are similar to the ichnogenus

Asianopodus . Unfortunately, the horizon yielding Asianopodus type tracks was not indicated in detail. Le Loeuff et al. (2009) also illustrated a sketch of some theropod trackways from the Tha Uthen site and indicated a resemblance to Asianopodus .

Asianopodus is characterized as a small- to medium-sized tridactyl, mesaxonic and subsymmetrical track with a distinct bulbous heel impression (Matsukawa et al., 2005). The interdigital angle between II and IV is 42°–59° and the footprint length/width ratio is 1.38 to 1.63 (mean 1.48; Matsukawa et al., 2005). In the Tha Uthen specimens of the northern MANUSCRIPT outcrop, each track has indistinct metatarsophalang eal pad impressions, but a distinct bulbous heel impression is difficult to recognize because of poor preservation of the posterior part. The ichnological measurements of the Tha Uthen specimens are basically similar to those of Asianopodus . Because of these morphological differences, the Tha Uthen specimens at the northern outcrop are assigned tentatively to cf. Asianopodus in the present study (Fig. 7). ACCEPTED

4.2. Flattened possible theropod tracks

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Theropoda Marsh, 1881

Ichnogen. et sp. indet. (Fig. 8)

Material: Two trackways: T23 composed of 6 consecutive tracks and T35 composed of

3 discontinuous tracks. The original tracks and trackway remain in the field (Figs. 5 and 6).

Locality and horizon: Same as 4.1.

Description: The average footprint length and width of Tha Uthen trackway T23 (Fig. 8) are 17.4 and 14.2 cm, respectively (Table 1). The mean step and stride length are 91.0 and MANUSCRIPT 186.5 cm, respectively. The mean pace angulation is 176.5°. Tha Uthen trackway T35 is poorly preserved, mostly as round impressions or incomplete. The average footprint length and width of trackway T35 are 13.5 cm (maximum 15.5 cm) and 11.3 cm (maximum 11.5 cm), respectively. The mean step is 96.5 cm. The well-preserved trackway T23 shows tridactyl impressions and facultative bipedal walking gait. The trackway width of T23 is narrow and eachACCEPTED track is inwardly rotated. Most tracks of trackways T23 and T35 are flattened or round impressions with no distinct digital pad traces, and are circular to

ACCEPTED MANUSCRIPT semicircular in shape with an indistinct border to the three digits. Each digit impression is comparatively elongate. The footprint length/width ratio is 1.23 (maximum 1.28).

Comparison and discussion: Le Loeuff et al. (2009) remarked that the Tha Uthen specimen (T23) is very similar to ornithopod tracks from Japan described by Matsukawa et al. (2006), and they provisionally referred the Tha Uthen specimen to the ichnogenus

Caririchnium . However, Caririchnium has broad and quite blunt digits, and is also characterized by a bilobed heel (Fig. 8); thus, the Tha Uthen specimen cannot be identified as Caririchnium .

Ornithopod tracks are rare in the Khorat Group. Lockley et al. (2009) reported MANUSCRIPT Neoanomoepus sp., which was formed by a primitive small-sized ornithopod, at the Hin Lat

Pa Chad site in the Lower Cretaceous Phra Wihan Formation. However, those tracks show quadrupedal walking with five manual and four pedal digits. Kozu et al. (2014) reported one ornithopod track, a natural cast 19.8 cm long and 15.9 cm wide, from the Khok Kruat

Formation, but that specimen is larger than the Tha Uthen specimen and shows robust digit impressions andACCEPTED the metatarsophalangeal pad.

Xing et al. (2014a) reported small-sized possible ornithopod tracks from the Houcheng

Formation, Shangyi, China. The Shangyi specimens (Fig. 8: SYO1 and SYO2) are tridactyl

ACCEPTED MANUSCRIPT pes impressions with no manus impressions, and lack claw marks. The size (length SYO1:

12.1 cm, SYO2: 15.1 cm on average) and narrow trackway width are similar to the Tha

Uthen material. However, the Shangyi specimens show round and robust digit impressions, and the maximum length/width ratio (1.00) is smaller than in the Tha Uthen specimen.

The Tha Uthen trackways T23 and T35 lack manus impressions. From the point of view of the ichnological shape of the specimens, at first glance, it looks like trackway T23 and

T35 are imprinted by ornithopod. However we suggest that the Tha Uthen specimens T23 and T35 are tracks of small-sized theropod. Lockley and Xing (2015) made a comparative review of flattened tracks which are imprinted by theropod. According to them, the lack of MANUSCRIPT discrete digital pads and inter-pad creases makes the tracks appear more like those of ornithopods than theropods. However the trackway pattern remains characteristically theropodan. Additionally, flattened tracks of theropod often reveal digit III with distinctive, diamond- or rhomb-shaped outlines (Fig. 8). In the Tha Uthen specimens T23 and T35, the tracks lack digital pads and inter-pad creases. The tracks also show high footprint length/width ratio,ACCEPTED narrow trackway width, and the impressions of digit III appear to widen distally (Fig. 8). Thus, the trackmakers of T23 and T35 are estimated to have been a small-sized theropod. It is impossible to give those tracks ichnological names because the

ACCEPTED MANUSCRIPT tracks are in a poor state of preservation. In comparison with other Asianopodus type theropod tracks, the tracks of T23 and T35 are relatively large, and shows solitary pattern.

Thus, the trackmakers of T23 and T35 are different with those of other Asianopodus type tracks.

5. Interpretation of the trackway assemblage

5.1. Trackway and group distribution

A total of 584 theropod tracks referred to cf. Asianopodus are recognized in association MANUSCRIPT with unnamed theropod and crocodylomorph tracks in an area of 72.5 m 2 on the northern outcrop surface at the Huai Dam Chum track site (Figs. 5 and 6). This occurrence constitutes an example of high-density dinosaur tracks in the Cretaceous strata of Thailand.

As mentioned above, the Asianopodus type theropod tracks are separated into two groups because the tracks show two directions of migration, to the northeast and to the northwest.

We define the groupACCEPTED aligned NW as Group A and the group aligned NE as Group B (Fig. 5).

Table 2 lists the estimated hip height and speed of the cf. Asianopodus trackmakers

(Thulborn, 1982, 1989). The mean estimated hip heights of Group A and B are 61.7 and

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58.3 cm, respectively; the speeds of those groups are estimated as 8.04 and 8.65 km/h, respectively (Table 2). The relative stride length (stride length/hip height = SL/h) is indicative of the use of different gaits such as walking (<2), trotting (2 to 2.9), and running

(>2.9; Thulborn, 1990). According to these definitions, the relative stride lengths of Groups

A and B are 2.10 and 2.35, respectively, implying that the trackmakers of Group A and B were trotting.

Recent discoveries of multiple trackway sites indicate that many dinosaur groups were habitually gregarious (Lockley, 1991). As shown in Fig. 9, the cf. Asianopodus trackways are parallel or sub-parallel to each other with little overlap, and show small or irregular MANUSCRIPT “intertrackway spacing” (the lateral space between adjacent trackways). Barco et al. (2006) concluded that a dinosaur group moved in a pack comprising at least three waves, on the basis of the closeness of the parallel trackways and their superimpositions on the same substratum. In Fig. 9, we illustrate some well-defined trackways of Group A. The trackways imprinted by individuals of the same size are oriented parallel or sub-parallel to one another with little overlap,ACCEPTED and the intertrackway spacings are small and partially irregular. In addition, the estimated travelling speeds are similar to each other. In common with Barco et al. (2006), we follow the hypothesis that Group A moved in a single pack comprising

ACCEPTED MANUSCRIPT several waves. For Group B, there are low number of well-defined trackways and many isolated tracks because most tracks are overlapping (Fig. 10). Thus, on the basis of the ichnological measurement data, we could not describe those behavioural patterns in detail, but Group B was probably produced by a single group. Thus, the theropod trackways of

Groups A and B at the Huai Dam Chum site are considered to record patterns of gregarious behaviour. Two trackways consisting of the flattened possible theropod tracks (labelled C;

Table 1 and 2) show S to SE movement directions meaning solitary behaviour.

5.2. Trackmaker affinity and its herd structure MANUSCRIPT From the measurements of the track assemblage, Groups A and B are inferred to have been imprinted by the same type of small-sized theropod. Theropod remains are relatively poorly known in the Khok Kruat Formation. On the basis of isolated teeth, Buffetaut et al. (2005) indicated the existence of a small-sized theropod; however, little is known about the affinity of the indeterminate theropod. On the other hand, the ornithomimosaurian Kinnareemimus khonkaenensis ACCEPTED was described by Buffetaut et al. (2009) from the Lower Cretaceous Sao

Khua Formation of the Khorat Group. In general, ornithomimosaurs are the best-known example of gregarious dinosaurs. Although direct evidence is lacking, we consider that the

ACCEPTED MANUSCRIPT theropod tracks referred to cf. Asianopodus in the Huai Dam Chum site were imprinted by ornithomimosaurian dinosaurs.

From the quantitative community analysis, scatterplots of footprint length–width measurements of the tracks of Groups A and B at the Huai Dam Chum site were constructed

(Fig. 11). In Group A, the values of trackway are widely scattered (regression line: y =

0.4939 x + 7.9887); in contrast, the footprint values are clustered around the regression line

(y = 1.1225 x + 2.232). From those results, we constructed a histogram showing frequency–length measurements using the trackway data ( n = 66). The footprint length is related to the size of the trackmaker. In this case, the histogram exhibits an anomalous MANUSCRIPT bimodal distribution, whereas the herd structure of dinosaur footprint assemblages normally shows a monomodal distribution (e.g. Barco et al., 2006; Lockley et al., 2006a). In this study, we consider two hypotheses as explanations for this pattern: male–female differences and different growth stages. In general, footprint shape and morphology may not reflect diagnostic differences between genera or species, or sexual dimorphism (e.g. Farlow, 2001;

Myers and Fiorillo,ACCEPTED 2009). There is no direct evidence that the bimodal distribution of the size-frequency histogram of the Tha Uthen theropod tracks reflects sexual dimorphism.

Ichnotaxa do not correspond to the taxonomical classification based on bone fossils (Myers

ACCEPTED MANUSCRIPT and Fiorillo, 2009); thus, it is difficult to describe the trackmaker of the Tha Uthen theropod track at a lower taxonomic level and to estimate the seasonal periodicity of the Tha Uthen site in detail. However, this result indicates the possibility of a pair-bonded lifestyle or reproductive cycles in dinosaurs (Fiorillo et al., 2014). For the second hypothesis, the mean estimated hip height of the trackmakers of Group A is 61.7 cm (Table 2). If all of those theropod producers were juveniles, the size of mature or old individuals would have been comparable to that of large-sized theropods such as Ceratosauria, Megarosauria, and

Carcharodontosauria in the Lower Cretaceous (Aptian–Albian). It is highly unlikely that such large-sized theropods were living in a large-scale family. The trackmakers of the cf. MANUSCRIPT Asianopodus Group A were probably mainly two distinct ontogenetic age groups without juveniles. If the ichnological interpretations of sexual and age segregation in the Tha Uthen theropod tracks are correct, then small-sized theropods may have possessed a complex herd social construction, as is already known for sauropods and ornithopods. For Group B, the measurement data from the quantitative community analysis are insufficient to describe the herd structure. However,ACCEPTED on the basis of the tentative values, we consider that Group B was composed of the same type of small-sized theropods as Group A, and therefore probably had the same herd structure.

ACCEPTED MANUSCRIPT

6. Conclusions

At the Huai Dam Chum site, ~600 dinosaur footprints are preserved in a thin mud layer in the northern part of the outcrop. We provide the first evidence that the trackmakers of cf.

Asianopodus were probably gregarious dinosaurs at this site, although the flattened tracks of possible theropod cannot be given an ichnological name. On the basis of the ichnological evidence, the Asianopodus type theropod tracks are separated into two gregarious groups, MANUSCRIPT Groups A and B. The tracks in Group A show well-defined characteristics of gregarious behaviour such as parallel or sub-parallel trackways and small intertrackway spacing with little overlap. On the basis of the sedimentary structures, it is estimated that the dinosaurs of

Group A travelled along the course of a river. From the quantitative community analysis, the histogram of size-frequency measurements of Group A shows an anomalous bimodal distribution. ThereACCEPTED are two possible hypotheses to explain the distribution: male–female differences and trackmakers of different growth stages. The results of quantitative

ACCEPTED MANUSCRIPT community analyses demonstrate the presence of small-sized theropod dinosaurs showing gregarious behaviour in Thailand during the Early Cretaceous.

Acknowledgements

We thank Prof. Shinobu Ishigaki (Okayama University of Science) for useful comments, suggestions, and support. We are also grateful to staffs of Tha Uthen ward office for their cooperative support. We thank staffs of Department of Mineral Resources of Thailand for MANUSCRIPT help and support with field work.

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Figure captions

Figure 1. Index map showing the distribution of the Khorat Group and footprint sites (Original map is from DMR, 1999; Meesook, 2011).

Figure 2. Lithostratigraphic column and dinosaur ichnofauna of the Khorat Group (modified from Buffetaut et al., 2009; Buffetaut and Suteethorn, 2011; Meesook, 2011; Shibata et al., 2011, 2015; Kozu et al., 2016 ).

Figure 3. Locality map (A), and outcrop photograph of the Huai Dam Chum site (B). The outcrop is cropping out along the route 212, and footprint-bearing outcrops were covered by artificial roof and easily to access to observe footprints at the site.

Figure 4. Lithostratigraphic clumn of northern part the outcrop at the Huai Dam Chum site.

Figure 5. Mesh map of the north part of the outcrop of north part of the Huai Dam Chum site.

Figure 6. Photographs and sketch of tracks of the MANUSCRIPT Huai Dam Chum site. In the sketch and photograph, many theropod consecutive tracks and a few small-sized crocodylomorph tracks are imprinted with current-ripple marks on the thin mud layer.

Figure 7. Comparison chart of theropod tracks. (A–F) Photographs of pes tracks of Tha Uthen specimens T8n4, 5, T11n3, T14n2, T32n5, and T84n3; (A’–F’) Sketches of pes track of Tha Uthen specimens T8n4, 5, T11n3, T14n2, T32n5, and T84n3; (G) Sketch of genoholotypic track of Grallator parallelus (modified from Olsen et al., 1998); (H) Sketch of Therangospodus isp. (modified from Xing et al., 2011); (I) Sketches of small-sized track morphoype 2 from the Yanguoxia site (modified from Zhang et al., 2006); (J) sketch of Asianopodus pulvinicalxACCEPTED (modified from Matsukawa et al., 2005). Scale bars of A to F, A’ to F’, H, I and J are 10 cm. Scale bar of G is 2 cm.

Figure 8. Comparison chart of flattened tracks. (A) Photographs of pes tracks of Tha Uthen tacks T23; (A’) Sketches of pes track of Tha Uthen tracks T23; (B) Sketch of Caririchnium sp. TGUSE-DT1007 (modified from Matsukawa et al., 2005); (C) Sketch of ornithopod tracks SYO1-11L and SYO2-11R (modified from Xing et al., 2014); (D) Sketch of flattened 37

ACCEPTED MANUSCRIPT theropod tracks XY-T1-L2 and XY-T1-L3 (modified from Lockley and Xing, 2015). Arrows mean maximum width of cast of digit III. Scale bar is 10 cm.

Figure 9. Northern part of the mesh map at the Huai Dam Chum site. Most of theropod trackways of Group A are parallel or sub-parallel to each other, and show small or irregular intertrackway spacing, but some theropod trackways are overlapping each other. Two solitary trackways consisting of flattend tracks intersect at a right angle with theropod trackways of Group A. Black arrows indicate the directions of crocodylomorph trackways. Red arrow indicates the direction of movement of Group A.

Figure 10. Southern part of the mesh map at the Huai Dam Chum site. Most of theropod trackways of Group A are parallel or sub-parallel to each other, and show NW direction. Some theropod trackways of Group B are identified, but most of tracks are overlapping each other, and intersect at a right angle with tracks of Group A. Each arrow indicates the directions of movement of Group A and B, respectively.

Figure 11. Histograms of the measurement data in the Tha Uthen theropod tracks. (A and B) are bivariate plots of footprint length-width measurements of Group A and B, respectively. (C, E and D, F) are size-frequency histogram for MANUSCRIPT Group A and B, respectively. (G and H) are histograms of estimated speed of the trackmakers of Group A and B, respectively.

Table 1. Measurements of well-preserved tracks of theropod and flattened tracks of possible theropod at the Huai Dam Chum site.

Table 2. Measurements of trackways of the Huai Dam Chum site.

ACCEPTED

ACCEPTED MANUSCRIPT

Tw No. FP FL FW II-IV II-III III-IV Dir Step Stride PA TD T1 n1 R 10.5 9 55 30 25 N65W53.5 105 - 2 L 11 8.5 45 20 25 N58W 52.5 111.5 165 3 R 11.5 7.5 40 18 22 N62W 61 121.5 165 4 L 12.5 8 42 20 22 N62W 62 - 170 5 R 13 8 40 20 20N70W - - - Average 11.7 8.2 44.4 21.6 22.8 57.3 112.7 166.7 A T2 n1 L 13 10 42 20 22 N65W 66 135.5 - 2 R 17 12 59 24 35 N70W 70 139.5 175 3 L 16 12 50 23 27 N68W69.5 - 172 4 R 15 10.5 48 25 23 N70W - - - Average 15.3 11.1 49.8 23 26.8 68.5 137.5 173.5 A T3 n1 L 17.5 11.5 48 23 25 N63W 81 162.5 2 R 15.5 12 60 25 35 N70W 82 170 3 L 17 12 56 28 28 N66W Average 16.7 11.8 54.7 25.3 29.3 81.5 162.5 170 A T4 n1 L 12.5 10 39 19 20 N70W 60 120 MANUSCRIPT 2 R 12 9 63 35 28 N73W 60 122 177 3 L 13 9 44 22 22 N65W 60 176 4 R 12 10 61 30 31 N82W Average 12.4 9.5 51.8 26.5 25.3 60 121 176.5 A T5 n1 L 12.5 7.5 45 20 25 N74W 63 2 R 12.5 8.5 48 28 20 N77W Average 12.5 8 46.5 24 22.5 63 A T6 n1 R 10 9 55 27 28 N40E 58 2 L 12 8.5 43 23 20 N35E Average 11 8.8 49 25 24 58 B T7 n1 R 14 9 50 24 26ACCEPTED N66W 70 140 2 L 13.5 10 54 29 25 N65W 70 144 175 3 R 15 10.5 55 30 25 N75W 76 169 4 L 13 - 61 30 31 N70W Average 13.9 9.8 55 28.3 26.8 72 142 172 A ACCEPTED MANUSCRIPT

T8 n1 L 16 10.5 63 33 30 N50W 56.5 111.5 2 R 16 10.5 59 29 30 N60W 55 112.5 170 3 L 15 10.5 52 27 25 N55W 57.5 113 172 4 R 14 9.5 49 25 24 N67W 55.5 111.5 170 5 L 14.5 10.5 53 31 22 N68W 56 109.5 170 6 R 14.5 10 57 26 31 N70W 54 170 7 L 15 10 60 33 27 N60W Average 15 10.2 56.1 29.1 27 55.8 111.6 170.4 A T9 n1 L 16.5 11 52 30 22 N55W 79 154.5 2 R 16 12.5 57 30 27 N53W 75.5 155 180 3 L 15.5 11 53 28 25 N65W 79.5 174 4 R 16 11 50 27 23 N58W Average 16 11.4 53 28.8 24.3 78 154.8 177 A T10 n1 L 17.5 13 53 28 25 N50W 82.5 164 2 R 16.5 12.5 60 29 31 N70W 81.5 160.5 177 3 L 15.5 12.5 60 33 27 N63W 79 158 175 4 R 16 13 63 30 33 N65W 80 155.5 170 5 L 16.5 13 61 33 28 N62W 76 MANUSCRIPT 175 6 R 14.5 11 56 30 26 N60W Average 16.1 12.5 58.8 30.5 28.3 79.8 159.5 174.3 A T11 n1 L 13.5 10 - - - N84W 76.5 153.5 2 R 13 9 41 19 22 N80W 77 174 3 L 12 10 55 27 28 N78W Average 12.8 9.7 48 23.0 25.0 76.8 153.5 174.0 A T14 n1 R 16.5 14 - - - N62W 74 146 2 L 15.5 11 47 32 15 N72W 72.5 170 3 R 16 11 48 26 22 N76W 4 R 15 11 44 16 28 N63W Average 15.8 11.8 46.3 24.7 21.7ACCEPTED 73.3 146 170 A T23 n1 R 17.5 14.5 - - - N172E 90.0 196.0 2 L 17.5 15 - - - N177E106.0 207.0 172 3 R 18.5 14 - - - N183E102.0 187.0 171 4 L 17 14.5 - - - N175E 86.0 156.0 175 ACCEPTED MANUSCRIPT

5 R 16.5 13 - - - N190E71.0 176 6 L 17.5 14 - - - N180E Average 17.4 14.2 --- 91.0 186.5 173.5 C T32 n1 L 12.5 N74W67 134 2 R 15 10.5 45 20 25 N68W 67 134 177 3 L 14.5 11 50 26 24 N68W 67 132 175 4 R 15 12 50 29 21 N64W 65 129.5 172 5 L 15 12 47 21 26 N65W 64.5 134 176 6 R 15 11 53 -- N70W 69.5 131 173 7 L 13 11 50 30 20 N78W 62 171 8 R 12.5 10 35 16 19 N74W Average 14.1 11.1 47.1 23.7 22.5 66.0 132.4 174 A T35 n1 L 15.5 11.5 - - - N205E 96.5 2 R 12.5 11.5 - - - N226E 3 R 12.5 11 - - - N208E Average 13.5 11.3 --- 96.5 C T84 n1 L 13.5 - - - 87.5171.5 2 R 16.511.5 - - 17 84 MANUSCRIPT 175 3 L 15.5 12 49 20 29 Average 15.2 11.8 49.0 20.0 23.0 85.8 171.5 175 B

Tw: trackway number, No.: footprint number, FP: foot part (L or R), FL: footprint length (cm), FW: footprint width (cm), II-IV: interdigital angle between II to IV, II-III: interdigital angle between II to III, III-IV: interdigital angle between III to IV, Dir: direction of digit axis III, Step: pace length (cm), Stride: stride length (cm), PA: pace angulation, TD: trackmaker description; A = theropod Group A, B = theropod Group B, and C = solitary theropod. ACCEPTED ACCEPTED MANUSCRIPT

Tw FL FW FL/FW h Step Stride PA S (m/s) S (km/h) TD T1 11.7 8.2 1.427 52.65 57.3 112.7 166.7 2.023 7.28 A T2 15.25 11.1 1.371 68.63 68.5 137.5 173.5 2.069 7.45 A T3 16.7 11.8 1.408 75.00 81.5 162.5 170 2.465 8.87 A T4 12.375 9.5 1.303 55.69 60.0 121 176.5 2.134 7.68 A T5 12.5 8 1.563 56.25 63.0 - - - - A T6 11 8.8 1.257 49.50 58.0 - - - - B T7 13.9 9.8 1.411 62.44 72.0 142 172 2.439 8.78 A T8 15 10.2 1.469 67.50 55.8 111.6 170.4 1.489 5.36 A T9 16 11.4 1.407 72.00 78.0 154.8 177 2.383 8.58 A T10 16.1 12.5 1.287 72.38 79.8 159.5 174.3 2.491 8.97 A T11 12.8 9.7 1.328 57.75 76.8 153.5 174 3.043 10.96 A T12 14.8 12 1.229 66.38 70.9 142 172 2.271 8.17 A T13 15.2 11.1 1.368 68.25 73.5 146.5 172.3 2.315 8.33 A T14 15.8 11.8 1.340 70.88 73.3 146 170 2.203 7.93 A T15 11.7 8.4 1.393 52.65 61.0 115.8 166 2.119 7.63MANUSCRIPT A T16 14.4 11.6 1.241 64.80 69.0 132.5 - 2.080 7.49 A T17 16 11.7 1.364 72.00 64.1 127.3 171 1.720 6.19 A T18 15.7 10.5 1.492 70.50 67.7 136 170.8 1.969 7.09 A T20 16.6 11.9 1.395 74.70 63.3 124.8 166 1.594 5.74 A T21 12.3 9.3 1.324 55.13 72.0 - - - - A T22 15.8 11.6 1.364 71.04 74.1 147.3 169.8 2.230 8.03 A T23 17.4 14.2 1.229 78.38 - 186.5 173.5 2.947 10.61 C T24 11.6 9.5 1.218 52.07 53.8 107.5 172.2 1.895 6.82 A T27 12.8 9.0 1.417 57.38 55.6 110.3 170.3 1.767 6.36 A T28 14.3 10.2 1.397 64.13 63.9 127.2 176 1.966 7.08 A T29 12.8 9.5 1.351 57.75 57.6 115.5ACCEPTED 174.8 1.893 6.81 A T30 14 9.7 1.440 63.00 60.9 120.4 173.3 1.831 6.59 A T31 14.8 10.3 1.438 66.50 66.7 134.6 171.6 2.073 7.46 A T32 14.1 11.1 1.270 63.28 66.0 132.4 174 2.137 7.69 A T33 15.5 11.6 1.341 69.75 63.8 126.9 184.5 1.776 6.39 A ACCEPTED MANUSCRIPT

T34 12.3 8.6 1.427 55.13 54.5 109.4 174.3 1.825 6.57 A T35 13.5 11.3 1.191 60.75 - 180.5 - 3.759 13.53 C T38 15.6 11.9 1.308 70.13 64.9 128.1 165.7 1.793 6.46 A T39 16.3 11.6 1.407 73.29 62.8 125 168.6 1.634 5.88 A T40 15 12.6 1.188 67.50 69.7 137.7 173.8 2.115 7.61 A T41 12.3 9.4 1.303 55.29 65.8 122.5 172.7 2.197 7.91 A T42 12.7 10.4 1.216 57.00 54.0 108.8 168 1.740 6.26 A T43 12.8 9.8 1.308 57.38 59.3 117.8 172.5 1.969 7.09 A T44 13.8 10 1.375 61.88 64.0 - - - - A T45 16.3 9.3 1.757 73.13 77.5 - - - - A T46 11.3 7.7 1.478 51.00 53.3 106.5 175 1.911 6.88 A T47 15.3 10.4 1.470 68.63 68.2 134.8 168 2.001 7.20 A T48 13.5 10.3 1.317 60.75 61.0 - - - - A T49 13 10.3 1.268 58.50 70.0 - - - - A T50 12.6 8.8 1.438 56.63 61.9 123.6 170 2.169 7.81 A T51 12.5 10.5 1.190 56.25 68.8 137.5 175 2.611 9.40 A T52 13.1 9.9 1.329 59.06 65.0 128.8 171 2.210 7.96MANUSCRIPT A T53 15.5 11 1.409 69.75 69.0 138.3 176 2.049 7.38 A T54 12.4 8.6 1.435 55.69 69.2 137.5 176.5 2.642 9.51 A T55 10.5 7 1.500 47.25 49.3 98 170 1.819 6.55 A T56 12.7 10.1 1.257 57.15 69.0 137 171.7 2.548 9.17 A T57 10.7 8.2 1.301 48.00 60.5 124 178 2.646 9.52 A T58 12.3 9.7 1.271 55.50 73.3 148 178 3.000 10.80 A T59 11 8.3 1.333 49.50 63.7 129 171.5 2.726 9.81 A T60 15.7 12.3 1.280 70.59 54.9 104.9 172.5 1.275 4.59 A T61 12.4 9.8 1.269 55.69 68.5 136.3 178 2.602 9.37 A T62 16.5 12.2 1.356 74.25 57.7 113.5 171 1.370 4.93 A T63 13 10.2 1.279 58.50 83.0 166ACCEPTED 170 3.416 12.30 A T64 13.8 10.2 1.361 62.25 64.8 129 170 2.085 7.51 A T65 11.5 8.75 1.314 51.75 51.5 - - - - A T66 15.5 12 1.292 69.75 82.5 - - - - A T67 15.3 11.1 1.371 68.63 62.2 126 171 1.788 6.44 A ACCEPTED MANUSCRIPT

T68 11 8.7 1.269 49.50 68.8 157 175 3.785 13.62 A T69 11.6 9.5 1.224 52.31 65.0 131.8 175 2.647 9.53 A T70 13.3 10.5 1.262 59.63 66.5 - - - - A T71 13.2 10.7 1.234 59.25 54.8 109 170 1.667 6.00 A T72 13.2 10.1 1.300 59.25 56.8 114 176.5 1.797 6.47 A T73 11 9.1 1.205 49.50 57.3 113.5 171.5 2.201 7.93 A T74 12.3 10.8 1.140 55.13 75.0 - - - - A T75 13.5 10 1.350 60.75 66.5 - - - - B T76 11.3 9.7 1.172 51.00 64.0 128.5 175 2.616 9.42 B T77 14.8 12.3 1.204 66.38 69.5 - - - - B T78 12 9.1 1.319 54.00 56.8 114.8 171.7 2.028 7.30 B T79 14.8 11.2 1.317 66.38 63.8 127.2 171 1.888 6.80 B T80 13.8 10.3 1.341 61.88 71.5 145.8 175.5 2.575 9.27 B T81 12.7 10.2 1.252 57.30 70.8 141 174 2.665 9.59 B T82 11.8 9.6 1.221 52.88 58.0 123 - 2.331 8.39 B T83 16.1 12.8 1.265 72.56 70.7 144.8 171.5 2.112 7.60 B T84 15.2 11.8 1.291 68.25 85.8 171.5 175 3.012 10.84MANUSCRIPT B T87 10 7.5 1.333 45.00 - - - - - B T88 11.5 8.5 1.353 51.75 - - - - - B

Tw: trackway number, FL: mean footprint length (cm), FW: mean footprint width (cm), FL/FW: mean footprint length/width ratio, h: mean hip height (cm), Step: mean pace length (cm), Stride: mean stride length (cm), PA: mean pace angulation, S (m/s): mean speed in s/h, S (km/h): mean speed in km/h, TD: trackmaker description; A = Group A, B = Group B, and C = solitary theropod. ACCEPTED ACCEPTED MANUSCRIPT

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Dinosaur footprint asemblage from the lower Cretaceous Khok Kruat Formation, Khorat

Group, northeastern Thailand

Shohei Kozu a, *, Apsorn Sardsud b, Doungrutai Saesaengseerung b, Cherdchan Pothichaiya b,

Sachiko Agematsu a, Katsuo Sashida a a Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki

305-8572, Japan b Department of Mineral Resources, Ministry of Natural Resources and Environment,

Thailand

Corresponding author. E-mail address: [email protected], Tel: MANUSCRIPT +81-29-853-6138

Hilights

-Small-sized theropod tracks from the Huai Dam Chum tracksite ichnotaxonomically described. ACCEPTED

-Well-defined characteristics of gregarious behaviour.

-Theropod tracks show nomalous bimodal distribution of well-defined characteristics.